It is well established that there is a constantly evolving need for new materials for optical devices and that the demands for quality are particularly high in the case of single crystals used in optical devices.
Recently there has been an increasing demand for materials that allow for the manipulation of light in the near UV, the UV and the deep UV. This region is roughly defined as light with wavelengths between 150 nm and 350 nm.
A particular need in this region is for coherent radiation with a completely solid state source. A fully solid state laser is desirable because such are generally compact, reliable, and rugged, with low power demands. In general, all-solid-state lasers capable of direct emission of coherent radiation in the UV region are not yet readily available.
Instead, the systems currently in use are very expensive, containing complex optical components that focus and refine coherent radiation.
A potentially superior alternative is to use IR diode laser sources to excite lasing ions such as Nd:YAG which emit in the IR (e.g. 1064 nm) and then employ a non-linear optical (NLO) crystal to generate second, third or fourth harmonics to multiply the frequency of the coherent radiation.
It is the acentricity of such NLO crystals that enables them to alter coherent radiation of one or more frequencies by frequency doubling or sum frequency generation. Thus, for example, two photons of 1064 nm wavelength can be frequency doubled by an appropriate acentric crystal to form one photon of 532 nm radiation. This ability to frequency sum and divide is a very important source of new coherent radiation wavelengths. The ability to alter the frequency of coherent radiation is generally referred to as non-linear optical (NLO) behavior. The general requirements of suitable NLO crystals are that they must form in an acentric structure and should preferably exist in a uniaxial structure for phase matching purposes. They should also be large enough (typically greater than 1×2×2 mm) for polishing and orientation in devices. They should also be of high optical quality, namely containing few impurities, defects or twinning
The most common inorganic crystals currently employed for NLO applications are K(TiO)PO4 (commonly referred to as KTP) and LiNbO3 (commonly referred to as LN). Both materials exhibit suitable NLO behavior in the visible region but their bandgaps are too narrow to exhibit satisfactory NLO behavior below 400 nm. Thus, there is a current demand for materials that have very wide bandgaps but display NLO behavior so they are suitable for use in the UV region as defined above. This is particularly the case for the next generation of ultra high-resolution photolithography. In this field it is anticipated that there will be demands for a wide variety of optical components capable of manipulating photons in the region between 120 nm and 200 nm. The list of demands for suitable UV NLO materials is well known. The crystals must be in an acentric space group for harmonic generation, have bandgaps wider than 200 nm (between 120 and 200 nm), good thermal stability and a very high optical damage threshold. Finally, they must be able to be grown as optical quality single crystals greater than several millimeters in size.
The primary class of compounds exhibiting this behavior is the metal borates. Borates generally have wide bandgaps (175-300 nm), high optical damage thresholds, and show a marked tendency to crystallize in acentric space groups. Thus, they are excellent materials for non-linear optical applications involving lasers with wavelengths below about 300 nm. Specifically, borates have recently received attention as potential NLO materials in the near UV, UV and deep UV. Several borate materials have recently been employed in commercial applications in UV optical devices. These include beta barium borate (β-BaB2O4, commonly referred to as BBO), LiB3O5 (commonly referred to as LBO) and CsLiB6O10 (commonly referred to as CLBO). Several other borates have also been proposed as potential UV NLO materials including Sr2Be2B2O7 (commonly referred to as SBBO) and YCaOBO3 (commonly referred to as YCOB). The primary limitation for full-scale employment of borate materials is based on the ability to grow high quality crystals of sufficient size for use in a device containing a coherent light beam. Borates often either decompose when they melt or, instead, tend to form highly viscous melts. These factors inhibit growth of good quality single crystals. The primary methods of growth are typically flux or stop seeded solution techniques requiring very high temperature. However, it is often difficult or impossible to grow large borate crystals of sufficient optical quality by either method.
Very small microcrystallites of tetragonal lithium borate of the formula LiBO2 with a uniaxial acentric structure have been formed by the simultaneous application of extremely high temperatures and pressures. These microcrystallites were incapable of scaleup to useful products, as they have dimensions of less than 1 mm on any side. Accordingly, this technologically demanding method prevents any foreseeable route to crystals of useful size or quantity.
Hydrothermal techniques are an excellent route to high quality single crystals for electro-optic applications. For example, all electronic grade quartz is grown commercially by the hydrothermal method. Further, KTP is grown by both flux and hydrothermal methods, and it is widely acknowledged by those familiar with the art that the hydrothermally grown product is generally of superior quality. The hydrothermal method involves the use of superheated water (liquid water heated above its boiling point) under pressure to cause transport of soluble species from a nutrient rich zone to a supersaturated growth zone. Generally, a seed crystal is placed in the growth zone to serve as a deposition site for growth, and supersaturation is achieved by the use of differential temperature gradients. The superheated fluid is generally contained under pressure, typically 5-30 kpsi, in a metal autoclave. Depending on the chemical demands of the system the autoclave can be lined with a noble metal using either a fixed or floating liner. These general techniques are well known to those of ordinary skill in the art and have been used for the growth of other electro-optic crystals, such as KTP and quartz.
Another need in deep UV optical component applications is for new grit material for grinding optical windows, mirrors, prisms and other optical components. Photonic applications in the deep UV, particularly, but not limited to 157 nm photolithography, require high quality material with bandgaps wider than 157 nm. The current materials of choice in these applications are CaF2 and MgF2. Both have very wide bandgaps but are somewhat soft and require extensive polishing for suitable use in photonic applications. The choice of polishing grit is particularly important because the softness of the fluorides invariably causes small amounts of grit to become embedded in the material resulting in imperfections in, for example, a finished photolithography map. Thus, it is important to use a very hard material with a bandgap greater than the wavelength in use. Finding a suitable grit for grinding wide bandgap optical components is difficult because few hard material such as nitrides, carbides, borides or metal oxides have a sufficiently wide bandgaps, while metal halides are rarely of sufficient hardness to serve as grit.